Signals

Django includes a “signal dispatcher” which helps allow decoupled applications get notified when actions occur elsewhere in the framework. In a nutshell, signals allow certain senders to notify a set of receivers that some action has taken place. They’re especially useful when many pieces of code may be interested in the same events.

Django provides a set of built-in signals that let user code get notified by Django itself of certain actions. These include some useful notifications:

weak – Django stores signal handlers as weak references by default. Thus, if your receiver is a local function, it may be garbage collected. To prevent this, pass weak=False when you call the signal’s connect() method.

dispatch_uid – A unique identifier for a signal receiver in cases where duplicate signals may be sent. See Preventing duplicate signals for more information.

Let’s see how this works by registering a signal that gets called after each HTTP request is finished. We’ll be connecting to the request_finished signal.

Receiver functions

First, we need to define a receiver function. A receiver can be any Python function or method:

def my_callback(sender, **kwargs):
print("Request finished!")

Notice that the function takes a sender argument, along with wildcard keyword arguments (**kwargs); all signal handlers must take these arguments.

We’ll look at senders a bit later, but right now look at the **kwargs argument. All signals send keyword arguments, and may change those keyword arguments at any time. In the case of request_finished, it’s documented as sending no arguments, which means we might be tempted to write our signal handling as my_callback(sender).

This would be wrong – in fact, Django will throw an error if you do so. That’s because at any point arguments could get added to the signal and your receiver must be able to handle those new arguments.

Connecting receiver functions

There are two ways you can connect a receiver to a signal. You can take the manual connect route:

Now, our my_callback function will be called each time a request finishes.

Where should this code live?

Strictly speaking, signal handling and registration code can live anywhere you like, although it’s recommended to avoid the application’s root module and its models module to minimize side-effects of importing code.

In practice, signal handlers are usually defined in a signals submodule of the application they relate to. Signal receivers are connected in the ready() method of your application configuration class. If you’re using the receiver() decorator, simply import the signals submodule inside ready().

Connecting to signals sent by specific senders

Some signals get sent many times, but you’ll only be interested in receiving a certain subset of those signals. For example, consider the django.db.models.signals.pre_save signal sent before a model gets saved. Most of the time, you don’t need to know when any model gets saved – just when one specific model is saved.

In these cases, you can register to receive signals sent only by particular senders. In the case of django.db.models.signals.pre_save, the sender will be the model class being saved, so you can indicate that you only want signals sent by some model:

The my_handler function will only be called when an instance of MyModel is saved.

Different signals use different objects as their senders; you’ll need to consult the built-in signal documentation for details of each particular signal.

Preventing duplicate signals

In some circumstances, the code connecting receivers to signals may run multiple times. This can cause your receiver function to be registered more than once, and thus called multiple times for a single signal event.

If this behavior is problematic (such as when using signals to send an email whenever a model is saved), pass a unique identifier as the dispatch_uid argument to identify your receiver function. This identifier will usually be a string, although any hashable object will suffice. The end result is that your receiver function will only be bound to the signal once for each unique dispatch_uid value:

Defining and sending signals

Defining signals

All signals are django.dispatch.Signal instances. The providing_args is a list of the names of arguments the signal will provide to listeners. This is purely documentational, however, as there is nothing that checks that the signal actually provides these arguments to its listeners.

Sending signals

To send a signal, call either Signal.send() (all built-in signals use this) or Signal.send_robust(). You must provide the sender argument (which is a class most of the time) and may provide as many other keyword arguments as you like.

Both send() and send_robust() return a list of tuple pairs [(receiver, response), ... ], representing the list of called receiver functions and their response values.

send() differs from send_robust() in how exceptions raised by receiver functions are handled. send() does not catch any exceptions raised by receivers; it simply allows errors to propagate. Thus not all receivers may be notified of a signal in the face of an error.

send_robust() catches all errors derived from Python’s Exception class, and ensures all receivers are notified of the signal. If an error occurs, the error instance is returned in the tuple pair for the receiver that raised the error.

The tracebacks are present on the __traceback__ attribute of the errors returned when calling send_robust().